Im having the same issue. did you ever figure this out? if so i would love to know how.
Are all blueprints provisioning from a VM tempalte? Is this governed by the size of the disk on the template?
Or, have there been machines provisioned from the Blueprints you cant change already. Remember the blueprint is also a management container so maybe you cant change from there and need to create a new blueprint?
Thanks. That was indeed the issue I had.